Cobalt-Free Ni-Base Superalloy

ABSTRACT

There is provided a Co-free Ni-base superalloy having a composition containing 1.0 to 10.0 wt % of Cr, 0.1 to 3.5 wt % of Mo, 7.5 to 10.0 wt % of W, 4.0 to 8.0 wt % of Al, 12.0 wt % or less of at least one species of Ta, Nb and Ti, 0 to 2.0 wt % of Hf, 0.1 to 5.0 wt % of Re and the remainder comprised of Ni and unavoidable impurities. The Co-free Ni-base superalloy exhibits high composition stability over a long period of time and is excellent in creep properties at high temperature, and also suitable as a turbine blade or turbine vane of atomic power generation in which maintenance for radioactive contamination and so forth are easy.

TECHNICAL FIELD

The present invention relates to an Ni-base superalloy, which is a heat resistant alloy, used for high-temperature apparatuses such as jet engines and gas turbines for industrial use. More specifically, the invention relates to a so-called cobalt-free Ni (nickel)-base superalloy, which does not contain Co (cobalt), the superalloy suitable as a turbine blade and turbine vane of atomic power generation and so forth.

BACKGROUND ART

An Ni-base superalloy is widely utilized as a material of a high-temperature apparatus because it is excellent in composition stability at high temperature and creep properties, and the patent applications of the same have been made (Patent Documents 1 and 2).

Although recently expected as a suitable material for such as a, turbine blade or turbine vane of atomic power generation, an Ni-base superalloy which is excellent in heat resistance contains a large amount of Co (cobalt). Co has excellent functions in that it enlarges the solubility limit at high temperature to the γ parent phase of Al, Ta, and the like, and also improves high-temperature strength by heat treatment for the dispersion deposition of a fine gamma prime phase, and thus is thought to be an essential component for an Ni-base superalloy used at high temperature. However, if Ni-base superalloy containing Co is polluted by radioactivity, the maintenance is extremely troublesome because Co has a long half life. Thus, for using an Ni-base superalloy as a member of a high-temperature apparatus of atomic power generation and so forth that may be possibly contaminated by radioactivity, the realization of an Ni-base superalloy not containing Co having a long half life is desired that has creep strength properties equal to or more than that of an Ni-base superalloy containing Co.

-   Patent Document 1: U.S. Pat. No. 5,366,695 -   Patent Document 2: European Patent No. 1,262,569

DISCLOSURE OF INVENTION

The present invention is made in consideration of the background as described above, and an object is to provide a cobalt-free (not containing Co) Ni-base superalloy that is suitable for a turbine blade and turbine vane of atomic power generation and the like, exhibits high composition stability over a long period of time and is excellent in creep properties at high temperature.

In order to solve the above problem, first, there is provided an Ni-base superalloy that has a composition containing 1.0 to 10.0 wt % of Cr, 0.1 to 3.5 wt % of Mo, 7.5 to 10.0 wt % of W, 4.0 to 8.0 wt % of Al, 12.0 wt % or less of at least one species of Ta, Nb and Ti, 0 to 2.0 wt % of Hf, 0.1 to 5.0 wt % of Re and the remainder comprised of Ni and unavoidable impurities.

Second, there is provided an Ni-base superalloy that has a composition containing 4.0 to 6.0 wt % of Cr, 1.0 to 3.0 wt % of Mo, 7.6 to 8.5 wt % of W, 4.5 to 6.0 wt % of Al, 4.0 to 10.0 wt % or less of at least one species of Ta, Nb and Ti, 0.1 to 1.6 wt % of Hf, 1.5 to 3.5 wt % of Re and the remainder comprised of Ni and unavoidable impurities.

Third, the above Ni-base superalloy is characterized in having in its composition one or more species of 0.3 wt % or less of Si, 3 wt % or less of V, 3 wt % or less of Zr, 0.3 wt % or less of C, 0.2 wt % or less of B, 0.2 wt % or less of Y, 0.2 wt % or less of La and 0.2 wt % or less of Ce.

Forth, a method of manufacturing a Ni-base superalloy characterized in that any of the above Ni-base superalloy is cast by a normal casting method, a one-direction solidifying method or a single crystal solidifying method.

Fifth, a method of manufacturing an Ni-base superalloy, characterized in performing a preliminary heat treatment at 1260 to 1300° C. for 20 minutes to 2 hours after casting, and performing a solution treatment at 1300 to 1350° C. for 3 to 10 hours, a first aging treatment at 1050 to 1150° C. for 2 to 8 hours and a second aging treatment at 800 to 900° C. for 10 to 24 hours.

Sixth, a turbine blade or turbine vane part, characterized in that any Ni-base superalloy described in any of the above is at least a part of its structure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the results of the comparison of creep lives of the existing strongest Ni-base superalloy containing Co and the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention has the characteristics as described above, and an embodiment will be set forth in detail hereinafter.

Co has functions of increasing the solubility limit at high temperature to the γ-parent phase such as an Al and Ta and also improving high-temperature strength by heat treatment for the dispersion deposition of a fine gamma prime phase. Thus, Co has been thought to be essential for an Ni-base superalloy excellent in composition stability at high temperature and creep properties. However, in the present invention, even without adding Co that has been thought to be essential for a highly strong Ni-base superalloy, by making an Ni-base superalloy of a specific composition, i.e., a composition containing 0 to 10.0 wt % of Cr, 0.1 to 3.5 wt % of Mo, 7.6 to 10.0% of W, 4.0 to 7.0 wt % of Al, 12.0 wt % or less of at least one species of Ta, Nb and Ti, 0 to 2.0 wt % of Hf, 0.1 to 5.0 wt % of Re and the remainder comprised of Ni and unavoidable impurities, it is possible to form a Ni-base superalloy having a high creep strength even as compared with CMSX-4 containing Co that has been used as a second generation Ni single crystal alloy.

In addition, in the present invention, depending on specific applications of a high-temperature apparatus using a Ni-base superalloy, for example, by adding one or more species of 0.3 wt % or less of Si, 3 wt % or less of V, 3 wt % or less of Zr, 0.3 wt % or less of C, 0.2 wt % or less of B, 0.2 wt % or less of Y, 0.2 wt % or less of La and 0.2 wt % or less of Ce, it is possible to improve physical properties of a product depending on various applications.

A cobalt-free Ni-base superalloy of the present invention is excellent in composition stability at high temperature and creep properties, and particularly suitable for the production of turbine blade or turbine vane parts.

Optimum content ranges suitable for components of a Ni-base superalloy of the present invention will be indicated below.

Cr (chromium) is an element excellent in oxidation resistance and improves a corrosion resistance of an Ni-base superalloy at high temperature. The content of Cr preferably ranges from 1.0 to 10.0 wt %, more preferably from 4.0 to 6.0 wt %.

Mo (molybdenum) is solid-solved in basis material to increase the high-temperature strength and also contributes to the high-temperature strength by deposition curing. The content of Mo is preferably in the range of 0.1 to 3.5 wt %, more preferably from 1.0 to 3.0 wt %.

W (tungsten) has an action of solid solution strengthening and deposition curing similar to Mo. The content of W is preferably from 7.5 to 10.0 wt %, more preferably from 7.6 to 8.5 wt %.

Al (aluminum) forms an intermetal compound expressed by Ni₃Al constituting a gamma prime phase, which binds to Ni to deposit in the gamma parent phase, in a ratio of from 50 to 70% by volume polarization and improves the high temperature strength. The content of Al is preferably in the range of 4.0 to 8.0 wt %, more preferably from 4.5 to 6.0 wt %.

Particularly in the present invention, Ta (tantalum), Nb (niobiumn) and Ti (titanium) all strengthen the gamma prime phase to improve the creep strength. One or more of the elements need to be added preferably at 0.1 wt % or more. In addition, when the total sum of the content of elements is 12 wt % or more, the formation of a harmful phase is accelerated, so the content should be 12 wt % or less. Further, the content is more preferably in the range of 4.0 to 10.0 wt %. Hf (hafnium) has an effect of improving oxidation resistance. When the content exceeds 2 wt %, the formation of a harmful phase is accelerated, so the content needs to be 2 wt % or less. Additionally, the turbine blade or turbine vane parts produced by a single crystal solidification method may have 0 wt % of Hf. more preferably from 0.1 to 1.6 wt %.

Re (rhenium) has effects of not only solid-solving in a gamma phase to improve the high-temperature strength by solid solubility strengthening, but also improving corrosion resistance. However, if Re is contained in a large amount, the TCP phase deposits at high temperature to possibly decrease the high-temperature strength. Thus, the content of Re is preferably in the range of 0.1 to 5 wt %, more preferably 1.5 to 3.5 wt %.

Si (silicon) forms a SiO₂ film on an alloy surface as a protection film and to improve oxidation resistance. However, if Si is contained in a large amount, it lowers the solid solubility limit of the other element, so the content of Si is preferably 3 wt % or less.

V (vanadium) is solid-solved in a gamma prime phase to strengthen the gamma prime phase. However, an excess content decreases the creep strength, so the content is preferably 3 wt % or less.

Zr (zirconium) strengthens the grain boundary like B (boron) and C. However, an excess content decreases the creep strength, so the content is preferably 3 wt % or less.

C (carbon) contributes to strengthening the grain boundary. However, an excess content inhibits the ductility, so the content is preferably 0.3 wt % or less.

B (boron) contributes, like C, to strengthening the grain boundary. However, an excess content inhibits the ductility, so the content is preferably 0.2 wt % or less.

Y (yttrium), La (lantern) and Ce (cerium) improve adhesion of a protection oxidation film forming alumina, chromia or the like during the use of the Ni-base superalloy at high temperature. However, an excess content lowers the solid solubility limit of the other elements, so preferably the content of Y is 0.2 wt % or less, the content of La is 0.2 wt % or less, and the content of Ce is 0.2 wt % or less.

A Ni-base superalloy of the present invention having the element composition as described above can be cast. Then, in this casting, an Ni-base superalloy can be produced as a polycrystalline alloy, a one-direction solidified alloy or l single crystalline alloy, for example, by a normal casting method, a one-direction solidifying method, or a single crystal solidifying method. A normal casting method basically casts a crystal by use of an ingot prepared to have a desired composition, and entails raising the casting temperature to about 1500° C. or more of solidified temperature of the alloy by heating, casting a superalloy, and then, for example, gradually moving the alloy away from a heating furnace to give temperature gradient thereto to grow many crystals in one direction. The single crystal solidification method is almost the same as the one-direction solidification method and includes providing a sector of a zigzag or spiral type prior to solidification of a desired article and making many crystals solidified in the one direction one crystal in the sector to produce a desired article.

An Ni-base superalloy of the present invention is subjected to heat treatment after casting to obtain a high creep strength. A standard heat treatment entails pre-heat treating an alloy at 1260 to 1300° C. for 20 minutes to 2 hours, and then heating the resulting material in the temperature range of 1050 to 1150° C. changed from the range of 1300 to 1350° C. for 2 to 8 hours, and air cooling. This treatment can be used together with coating treatment for the purpose of obtaining heat resistance and oxidation resistance. After air cooling, a subsequent second aging treatment for the purpose of sigma prime stabilization is carried out at 800 to 900° C. for 10 to 24 hours, and then air cooling is applied thereto. Each air cooling may be replaced with inert gas cooling. An Ni-base superalloy produced by the manufacturing method realizes high-temperature parts of a turbine blade or turbine vane of a gas turbine and the like.

EXAMPLES

Nine different kinds of samples (No. 1 to No. 12) of the compositions of Table 1 were cast to single crystals by a usual method and the crystals were subjected to liquidation treatment and aging treatment. In the liquidation treatment, the crystal was maintained at 1300° C. for one hour and then the temperature was raised to 1330° C. and held for 5 hours. In addition, the aging treatment included a first aging treatment that maintained a material at 1100° C. for 4 hours and a second aging treatment that maintained a material at 870° C. for 20 hours.

TABLE 1 (wt %) Sample No. Cr Mo W Al Ta Nb Ti Hf Re Ni No. 1 1.0 0.5 7.5 4.5 5.5 0 0 0 1.0 Remainder No. 2 4.5 1.8 8.0 4.9 0 1.5 0 0.1 2.2 Remainder No. 3 6.0 3.5 10.0 7.0 0 0 0.1 2.0 5.0 Remainder No. 4 1.0 0.5 7.5 4.5 0 1.5 0 0 1.0 Remainder No. 5 4.5 1.8 8.0 4.9 5.5 0 0 0.1 2.2 Remainder No. 6 6.0 3.5 10.0 7.0 0 1.5 0 2.0 5.0 Remainder No. 7 1.0 0.5 7.5 4.5 0 0 0.1 0 1.0 Remainder No. 8 4.5 1.8 8.0 4.9 0 0 0.1 0.1 2.2 Remainder No. 9 6.0 3.5 10.0 7.0 5.5 0 0 2.0 5.0 Remainder No. 10 4.0 1.0 7.6 8.0 6.0 1.5 0.5 0 0.1 Remainder No. 11 5.0 0.1 8.5 4.0 4.0 2.0 0 1.6 1.5 Remainder No. 12 5.0 3.0 8.5 6.0 0 3.5 0.5 0.1 3.5 Remainder

Next, creep strengths were measured for the samples of Example subjected to liquidation treatment and aging treatment. In the creep test, the life was defined as the time up to when a sample was ruptured by creep under the conditions of 800° C./735 MPa, 900° C./392 MPa, 1000° C./245 MPa and 1100° C./137 MPa. Additionally, in Example, the measurements were carried out for 9 different kinds of samples (No. 1 to No. 12). The samples of No. 1 to No. 12 did not exhibit a significant difference in test results. In FIG. 1, the comparison of the creep lives between the creep test results using the No. 5 sample in the present invention and using CMSX-4 is shown.

As can be seen in FIG. 1, the comparison of an Ni-base superalloy of the present invention not containing Co with CMSX-4 containing Co having been used as a second generation Ni single crystal alloy shows that the Ni-base superalloy of the invention has a creep strength equivalent to or higher than that of CMSX-4.

In the formation of a metal as a cobalt-free Ni-base superalloy (composition of No. 5 in Table 1) obtained in the present invention, a turbine blade and turbine vane were produced by generally used solidification method, single crystal solidification method and one-direction solidification method to measure the physical properties, respectively. It was confirmed that a turbine blade and turbine vane thus formed each exhibit high composition stabilities even at high temperature for a long period of time and are excellent in creep characteristics at high temperature.

INDUSTRIAL APPLICABILITY

According to the invention of the above first Ni-base superalloy, there can be provided alloys that are suitable for turbine blades and turbine vanes such as gas turbines for jet engines and power generation and keep good balances at medium temperatures to high temperatures. In particular, the Ni-base superalloy does not contain Co which has a long half life, and thus can be commercialized as material of atomic power generation and the like. In other words, it is possible to produce a cobalt-free Ni-base superalloy that is suitable as a turbine blade and turbine vane of atomic power generation and so forth, exhibits a high composition stability over a long period of time and is excellent in creep characteristics at high temperature.

According to the invention of the above second Ni-base superalloy, in addition to the above effects, further limitation of the composition can provide an alloy that is more suitable as a turbine blade and turbine vane and keep good balances at medium temperatures to high temperatures.

According to the invention of the above third to eighth turbine blades and turbine vanes, the cobalt-free Ni-base superalloy produced in claim 1 or 2 is formed by a normal solidification method, single crystal solidification method and one-direction solidification method to be capable of producing turbine blade or turbine vane parts that exhibit a high composition stability over a long period of time and are excellent in creep characteristics at high temperature. 

1-6. (canceled)
 7. An Ni-base superalloy, having a composition containing 1.0 to 10.0 wt % of Cr, 0.1 to 3.5 wt % of Mo, 7.5 to 10.0 wt % of W, 4.0 to 8.0 wt % of Al, 12.0 wt % or less of at least one species of Ta and Nb, 0 to 2.0 wt % of Hf. 0.1 to 5.0 wt % of Re and the remainder comprised of Ni and unavoidable impurities.
 8. The Ni-base superalloy of claim 7, having a composition containing 4.0 to 6.0 wt % of Cr, 1.0 to 3.0 wt % of Mo, 7.6 to 8.5 wt % of W, 4.5 to 6.0 wt % of Al, 4.0 to 10.0 wt % or less of at least one species of Ta and Nb, 0.1 to 1.6 wt % of Hf. 1.5 to 3.5 wt % of Re and the remainder comprised of Ni and unavoidable impurities.
 9. The Ni-base superalloy of claim 7, wherein the composition further contains at least one species of 0.3 wt % or less of Si and 0.5 wt % or less of Ti.
 10. A method of manufacturing a Ni-base superalloy, wherein any of the Ni-base superalloys of claim 7 is cast by a normal casting method, a one-direction solidifying method or a single crystal solidifying method.
 11. The method of manufacturing an Ni-base superalloy of claim 10, comprising: performing a preliminary heat treatment at 1260 to 1300° C. for 20 minutes to 2 hours after casting; and performing a solution treatment at 1300 to 1350° C. for 3 to 10 hours, a first aging treatment at 1050 to 1150° C. for 2 to 8 hours and a second aging treatment at 800 to 900° C. for 10 to 24 hours.
 12. A turbine blade or turbine vane part, wherein any Ni-base superalloy of claim 7 is at least a part of its structure. 